It will work because condensation will drain both ways, but I doubt that you would ever find an inspector who would approve it. They usually only know what is "in the book" and cannot visualize how anything else would work properly.

Here is my situation. The bathroom is in a dormer in the attic that has one wall that is an outside wall but there is a 2 feet crawl space on the bottom. So I was thinking of wenting from the sink go up then down then around to connect to the 2 inch meta venting that goes to the roof.

Yes, the cast iron is there, but I am nor sure how I can tap into the cast iron, especially since there are two cast iron pipes connected there, one 3 inch that is the drain from the toilet plus a 2 inch that is a vent from the lower floors.

I want to avoid going through the roof because of the fear that I may not do it properly and then there is a leak or I fall

if I can go around the small wall, which requires going below the highest fixture, then I can connect to the 2 inch metal venting on the oposite wall.

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You're going to have a tough time tying into the 3" CI stack at the proper height, so your best bet is to go through the roof... It's not a big deal...

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Seems the easiest way to go through the roof. One thing I have not drawn out on the picture is a plastic vent through the roof from a high efficiency furnace. Now, the guys who did it they connected the fresh air supply and the venting together to a wye and then one 3 inch pipe is going through the roof. Is that set up ok (mixing the supply and the condensation vent? If so, can I tap into that with the sink vent since it is plastic and easy to work with?

Does it have a tee currently where I am showing the red line tapping into it? If so, could you not remove the tee and plus a 4-way ( + ) one? And anyway, your proposed vent had it going over there anyway to that exact spot. I just cut out the extra wasted amount going up, across and back down.

sorry about the confusion in the drawing. A 2 inch cast iron ties with a 45 degree tee into the 3 inch cast iron. The propose venting was passing trough that area but it was not connected. I was planing of going pass the 3 inch cast iron then turn 90 degrees to go behind the low wall and conect on the oposite side where the 2 inch venting is. That's why I was asking about going below the highest fixture.

Ah, now I see. Besides the fact you'd have to go below the sink (not a good thing) you'd have to rip out the wall all the way around the bathroom and replace it to do it that way. While I'd be as hesitant as you about cutting a hole in my roof, I'd probably do that before I'd rip out sheet rock from 3/4 the way around the bathroom. Sounds to me like you either need to cut into that 3" iron pipe and put in a tee (which sounds like more trouble that it's worth) or go through the roof. Anything else would be kind of going beyond what's reasonable.

Keano, I don't typically give advice here since I'm new at this and still feeling my way through my own plumbing projects. But I just wanted to share with you my experience with CI as a DIYer.

In my case, I had a 4" CI vent to tap in to in order to install a Wye for a second toilet vent. The location of choice was in a closed wall, behind the toilet. Naturally, I had to remove the toilet and open the wall the gain access to the pipe, but it sounds like you're at that stage already.

Then, with nothing but a 4" angle grinder, I began cutting away at the pipe. The grinder easily got half way through at my cut marks, then I had to back out and slice a wedge out of the section I was removing. A couple of vertical cuts along the sides, and the front half of the piece I was removing practically fell out. This left me a big opening to get at the back half, which came out just as easily. The cuts were clean and smooth and straight. I slapped the no-hub fittings on there, slid the metal bands out of the way, rolled up the rubber coupling, and the wye slid right into place.

It was intimidating to start, but actually a very straight forward and satisfying project. Only two words of caution:
- Griding iron is extremely messy. Wear a mask. Wear goggles. Protect finished surfaces with plastic.
- Iron pipe is heavy. Make sure what you're cutting is supported -- both ends, before you get to cutting. Riser clamps for iron pipe are cheap, and will prevent an unsupported pipe from dropping through the floor or pulling through the roof.

Ah, now I see. Besides the fact you'd have to go below the sink (not a good thing) you'd have to rip out the wall all the way around the bathroom and replace it to do it that way. While I'd be as hesitant as you about cutting a hole in my roof, I'd probably do that before I'd rip out sheet rock from 3/4 the way around the bathroom. Sounds to me like you either need to cut into that 3" iron pipe and put in a tee (which sounds like more trouble that it's worth) or go through the roof. Anything else would be kind of going beyond what's reasonable.

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This is an attic space, I have access on both sides of the high walls, and there is space behind the small wall ( a duct runs behind that 2 ft wall) so I do not need to rip any sheetrock, the vent pipe would just go behind that wall.